CHUBBY and PAXOS

1. CHUBBYandPAXOS Sergio Bernales Dennis Kafura – CS5204 – Operating Systems

2. Overview • Chubby • Why? • How? • More precise how • Paxos • Why? • How? • More precise why Dennis Kafura – CS5204 – Operating Systems

3. 10 – 30,000 Clients Different client services Who is in charge? Guarantee it? What if you go down? i.e. DNS Server Dennis Kafura – CS5204 – Operating Systems

4. Service Requirements Easy for client developers (of the variety kind) Reliable and Available Master is clear –hint, hint Paxos Thousands of clients, tiny things Growing clients = growing transactions Dennis Kafura – CS5204 – Operating Systems

5. Chubby Coarse-grained lock service Caching Event Notification Access Control Lists /ls/foo/wombat/pouch Dennis Kafura – CS5204 – Operating Systems

6. System Structure Cache: data, metadata, absent file, handles (invalidated when data is changed) Dennis Kafura – CS5204 – Operating Systems

7. Files, Directories, Handles • /ls/foo/wombat/pouch • ACL • Controls: reading, writing, ACL changes • ACL authentication in RPC • Meta-data of node (64 bits entries) Dennis Kafura – CS5204 – Operating Systems

8. Sequencer • Sequence numbers provided only for interactionc that use locks • Request sequencer • Name of lock • Acquisition mode (exclusive, shared) • Lock generation number • Lock delay Dennis Kafura – CS5204 – Operating Systems

9. API (1 of 2) • Open(/ls/foo/wombat/pouch )* • Close() • Poison() • GetContentAndStats() • getStat() • ReadDir() • SetContents() • Compare and swap • Delete() • Acquire(), TryAcquire(), Release() Dennis Kafura – CS5204 – Operating Systems

10. API (2 of 2) GetSequencer() SetSequencer() CheckSequencer() Dennis Kafura – CS5204 – Operating Systems

11. Sessions • Keeps locks active • Data consistent • Survives failures • KeepAlives • Most calls are KeepAlives Dennis Kafura – CS5204 – Operating Systems

12. Events • Subcribe to • Contents modified • Child node added • Chubby master fail • Handle has become invalid • Lock acquired • Conflicting lock request Dennis Kafura – CS5204 – Operating Systems

13. Efficiency Atomic operations No current directories No last access times Proxies KeepAlivereplies used to transmit events and cache invalidations Dennis Kafura – CS5204 – Operating Systems

14. Back to the DNS Server Chubby caches Chubby batches Don’t use as fileserver Do use for config files, elect masters Dennis Kafura – CS5204 – Operating Systems

15. Master Fail - Over Dennis Kafura – CS5204 – Operating Systems

16. Recover Pick a new client epoch number(sent by clients) Master responds to master location requests Rebuilds session memory strucuture KeepAlives allowed Notifies clients of failure Master waits sessions syncs Fully operational While operating, handles are verified Ephemeral files removed Dennis Kafura – CS5204 – Operating Systems

17. Intro Paxos A consensus algorithm Only a value that has been proposed may be chosen Only a single value is chosen Unless value actually chosen, process don’t learn about it If someone disagrees, no one has agreed Dennis Kafura – CS5204 – Operating Systems

18. Paxos Proposers, Acceptors, Learners Multiples of each System assumes failure Data never corrupted Dennis Kafura – CS5204 – Operating Systems

19. Choosing a Value • P1- An acceptor must accept the first proposal it receives • Problem • Several values proposed, none chosen • Possible to accept multiple • A solution: number proposals Dennis Kafura – CS5204 – Operating Systems

20. Choosing a Value P2- If a proposal with value v is chosen, then every higher numbered proposal that is chosen has value v P2a- If a proposal with value v is chosen, then every higher accepted by any proposer has value v P2b- If a proposal with value v is chosen, then every higher accepted by any proposer has value v Dennis Kafura – CS5204 – Operating Systems

21. Choosing a Value P2c- For any v and n, if a proposal with value v and number n is issued, then there is a set S consisting of a majority of acceptors such that either (a) no acceptor in S has accepted any proposal numbered less than n, or (b) v is the value of the highest-numbered proposal among all proposals numbered less than n accepted by the acceptors in S P1a- An acceptor can accept a proposal, iff if has not reponded to a prepare request having a number greater than n Dennis Kafura – CS5204 – Operating Systems

22. Algorithm • Phase 1 • Proposer selects proposal number n and sends prepare request to majority of acceptors • If acceptor receives request number n greater than that of any other prepare request it has accepted, it sends back promise not to accept any more and also it’s last n • Phase 2 • Proposer receives prepare request form majority of acceptors, then sends the proposal number n plus the actual value v • If acceptor receives accept request for proposal n, accepts it only if it has not responded to another prepare request. Dennis Kafura – CS5204 – Operating Systems

23. Learn a Chosen Value • Chosen value propagated • All acceptors send messages to learners • Distinguished learner gets the message, passes it on • Distinguished learnerS • Learner ask acceptors • Distinguished Acceptors Dennis Kafura – CS5204 – Operating Systems